Group III-V semiconductor materials such as GaN and AlGaN are widely applied to optoelectronics due to advantages of a wide and controllable energy bandgap. Particularly, light emitting devices such as light emitting diodes or laser diodes using a Group III-V or Group II-VI semiconductor material can now render a variety of colors such as red, green, blue, and ultraviolet through development of thin film growth technologies and device materials. It may also be possible to produce white light at high efficiency using fluorescent materials or through color mixing. Furthermore, such light emitting devices have advantages such as low power consumption, semi-permanent lifespan, fast response time, safety, and environmental friendliness as compared to conventional light sources, such as fluorescent lamps and incandescent lamps.
Therefore, these light emitting devices are increasingly applied to transmission modules of optimal communication units, light emitting diode backlights as a replacement for cold cathode fluorescent lamps (CCFLs) constituting backlights of liquid crystal display (LCD) devices, lighting apparatuses using white light emitting diodes as a replacement for fluorescent lamps or incandescent lamps and headlights for vehicles and traffic lights.
FIG. 1 is a cross-sectional view illustrating a conventional light emitting device 100.
The conventional light emitting device 100 includes a substrate 110 formed of sapphire or the like, a light emitting structure 140 formed on the substrate 110 and including a first conductivity-type semiconductor layer 142, an active layer 144, and a second conductivity-type semiconductor layer 146, and a first electrode 152 and a second electrode 156 respectively disposed on the first conductivity-type semiconductor layer 142 and the second conductivity-type semiconductor layer 146.
The light emitting device 100 includes the active layer 144 in which electrons injected through the first conductivity-type semiconductor layer 142 meet holes injected through the second conductivity-type semiconductor layer 146 to emit light with energy determined by an intrinsic energy band of the material forming the active layer 144. Light emitted from the active layer 144 may vary according to the composition of the material constituting the active layer 144 and may be blue light, UV light, or deep UV light.
In the aforementioned light emitting device, particularly, in a horizontal light emitting device, light proceeding downward may be absorbed by the substrate 110 or undergo total reflection from an inner interface of the substrate 110, or the like, reducing light extraction efficiency.
In order to overcome this drawback, there is a need to reflect or scatter light at the surface of the substrate 110 to reduce total reflection of light.
FIG. 2A is a diagram illustrating air voids formed by a patterned substrate in a conventional light emitting device. FIG. 2B is a diagram illustrating a process of forming air voids in a conventional light emitting device by a wet etching process. FIG. 2C is a diagram illustrating functions of a mask of FIG. 2B.
In the light emitting device illustrated in FIG. 2A, air voids are formed at the surface of a buffer layer including AlN using a patterned sapphire substrate (PSS). In the light emitting device illustrated in FIG. 2A, a pattern and air voids are formed at the interface between the PSS and the buffer layer, so that light generated in a layer including GaN does not proceed into the PSS, but is instead scattered or reflected, thus improving light extraction efficiency of the light emitting device.
Referring to FIG. 2B, a mask 115 is formed on the surface of the substrate 110 including sapphire using a silicon oxide, and a light emitting structure 140 including GaN is grown thereon. Then, the silicon oxide (SiO2) is removed using hydrofluoric acid or the like. GaN is etched at areas adjacent to regions from which the silicon oxide has been removed to form air voids.
FIG. 2C illustrates functions of a mask formed of silicon oxide or the like. Crystal defects, which are formed at the interface between a substrate and a buffer layer and illustrated by vertical solid lines herein, may be blocked by the mask shown as red lines. The buffer layer growing between the masks laterally grows as illustrated by lateral arrows, so that the buffer layer may also be formed adjacent to mask regions.
However, the aforementioned conventional light emitting device has the following problems.
A process for optimizing size, period, and shape of the pattern formed in the PSS needs to be added to the process of manufacturing the light emitting device illustrated in FIG. 2A. In addition, in the light emitting device illustrated in FIG. 2B, a manufacturing process may become overcomplicated and manufacturing costs may increase due to deposition and patterning of the silicon oxide and the wet etching process.